Mechanoreceptors, Nociceptors, and Orthodontic Tooth Movement

The periodontal ligament contains rich sensory receptors including nociceptive free nerve endings and specialized mechanoreceptors. Morphologic investigations have demonstrated that the Ruffini ending, categorized by low-threshold stretch receptors, is an essential mechanoreceptor in the periodontal ligament among mammals. The periodontal mechanoreceptors (Ruffini endings) have an ability to respond to noxious stimuli and are also involved in nociception in addition to mechanoreception. In addition, peripherally released inflammatory mediators may increase the sensitivity of nociceptors and nociceptive inputs, which are relayed through the trigeminal spinal nucleus. Clinically, orthodontic tooth movement causes discomfort and/or abnormal sensations such as pain after the application of orthodontic forces. The abnormal sensations, usually momentary, disappear gradually. Many experimental studies have shown drastic alterations in the distribution and terminal formation of periodontal neural elements during experimental tooth movement. Further, experimental molar tooth movement has been shown to induce c-fos expression in the lateral parabrachial nucleus. Therefore, the temporal and spatial distribution pattern of Fos (Fos protein is a protein product of the c-fos oncogene) in the trigeminal subnucleus complex and the supraspinal structures may be involved with the pain during orthodontic tooth movement. There have been few reports on the physiological properties of periodontal mechanoreceptors and nociceptors during orthodontic tooth movement. This article reviews the current knowledge of the biological aspects of mechanoreceptors and nociceptors during orthodontic tooth movement.